For example, in a photolithography process for manufacturing a semiconductor element, and the like, an exposure apparatus for transferring a pattern on a master (mask or reticle) onto a substrate (wafer or glass plate) coated with a photoresist is used. Recently, a reduction projection exposure apparatus, which is advantageous in terms of resolution and aligning precision, has been used. Usually, a semiconductor element is manufactured by forming multilevel circuit patterns on a plurality of shot regions arrayed on a substrate by using different masters.
A shot array comprised of a plurality of shots arranged on the substrate will be called a shot layout. The shot layout is formed by considering the size of the substrate, the sizes of the shot regions, the sizes of the chip regions, and the like. A necessary number of shots selected from the shot layout in order to overlay circuits for forming the multilevel circuits described above is called sample shots. The sample shots are usually selected automatically. As a sample shot selecting method, methods disclosed in Japanese Patent Laid-Open Nos. 63-232324 and 63-232325 are known. The sample shot selecting method is important in overlay accuracy.
Conventionally, concerning selection of the sample shots, when an arbitrary number of shots is specified, the arbitrary number is arithmetically processed from the shot layout described above on the basis of the central coordinates of the shots arranged in the shot layout. As in the conventional case, when the shot size is comparatively small and the required precision is comparatively small and the required precision is comparatively moderate, the shot center and an alignment measurement mark position, which is used for actual overlaying, can be treated to be identical while causing no problems.
In recent years, the size of the semiconductor element increases, a region (angle of field) that can be exposed by one shot is widened in order to improve the productivity, and a requirement for improving the overlay accuracy has become stricter. Thus, an adverse influence produced by treating the shot center and the alignment measurement mark position to be identical becomes no longer negligible.
Conventionally, measurement marks are arranged for each measurement direction (X and Y directions) (X-direction marks are arranged along a transverse scribing line and Y-direction marks are arranged along a vertical scribing line), so a measurement error is minimized. However, in order to improve the productivity, marks with which the measurement time can be shortened, i.e., measurement marks with which both the X and Y directions can be measured simultaneously, are employed. Also, in order to correct the shot shape, multi-point measurement marks are arranged within the shot. This makes it difficult to improve the precision when sample shot selection is performed in the conventional manner by treating the shot center and the measurement mark position to be identical.
As described above, in conventional sample shot selection for performing overlaying, the arithmetic process is performed on the basis of the shot center. On the shot layout, even if the sample shots can be arranged in the vicinities of positions at an equal distance from the substrate center, when the positions of actual alignment measurement marks arranged in the shot do not correspond to the shot center, the measurement mark positions are deviated from the substrate center.
In fact, the overlaying measurement marks in the shot are rarely arranged at the shot center, and are usually formed in scribing line regions arranged between shots. Even if the centers of selected sample shots can be arranged at the equal distance from the substrate center, for example, the X/Y-direction simultaneous measurement mark positions are displaced form the shot center by half the shot size at minimum. This appears as a deviation amount from the substrate center.
In an ideal state, this deviation amount does not adversely affect the overly accuracy. In fact, however, a measurement error caused depending on the coating uniformity of the resist, and a position detection error such as expansion/contraction of the substrate, which is caused by annealing of the substrate, exist. When these errors exits and the deviation of measurement mark positions from the substrate center is present, an error occurs in a correction amount obtained by a statistical process, to adversely affect the overlay accuracy.
For example, assume that X/Y-direction simultaneous measurement marks are used. When the shot size is 20 mm and a substrate magnification component produced by manufacturing process factors is 5 ppm, a shift component produced by the overlaying measurement value is about 50 nm when compared to a case with no substrate magnification component. This cannot be neglected when compared to a recent required overlay accuracy of 30 nm.
It is known that the measurement error caused depending on the coating uniformity of the resist, and expansion/contraction of the substrate, which is caused by annealing of the substrate, occur radially from the substrate center and equally concentrically due to the manufacturing process. Also, the expansion/contraction in the radial direction varies in accordance with the manufacturing process and the density of the circuit pattern.
In order to decrease an influence on the overlay accuracy caused by the manufacturing process factors, the present invention has as its object to provide an automatic selecting method for alignment measurement sample shots, which is not adversely affected by the shot size.